Retina n' Testicle Question..:) - (Jul/28/2006 )
i know it might sound like a silly question...
Why are the photoreceptors on the "reverse" side of the retina???....past all the nerve and blood vessel layers!!!
Oh while u at it...may answer question as well...why the human testicles outside the body...where its prone to damage...after all we need that to pass on our genes!!!!
I have no idea about the first question, but I think the second is to facilitate independent temperature regulation...
I agree with Homebrew on the second answer.
But about the first not sure. Could b to do something with the way they function.
I had no idea either, so good question...
Try this "How the Retina Works," in American Scientist, Vol. 91, No. 1, Jan.-Feb. 2003, pp. 28-35.
Should be free online, I think.
The testicle thing is supposedly temperature regulation, allowing sperm maturation without mutation in structure, though there is no evidence that people with undescended testicles have any lower fertility than those with descended testicles.
Apparently sperm cells mature better at around 30 deg C rather than the 37 deg C of the rest of the body. Testicles outside the body help keep the temperature slightly lower than the core.
I am so sorry, I may not be able to answer ur first question, but for the second question, it is because outside the body, it provides a temperature at about 3 degree lower than normal body temperature, it is necessary for the production of healthy sperms.
I think it is or was a kind of gene defect, that happed in the phylogenetic history of mammals (they all have this problem, look at male dogs, pigs, etc, etc..., but i don't know what about more primitive mammals as egg laying ones). Other animals are not affected, nevertheless it was apparently not a working point of selection, mammalian ancestors were not less successful than the ones without defect and they did not become extinct. That's similar to other more or less important defects as the lost vitamin c production ability (I guess only primates?), which did not affect survival or at least not to far or only for some time. Perhaps mammals have enough behavioural and developmental plasticity to compensate. But nobody knows the truth.
Retina: It works and animals with this suboptimal type of eye were successful that is what counts. Probably because most animals (vertebrats) with this eye-type rely more on other senses as nose and ears and even sense of touch (eg rodents). Primates (and birds) are a special case as they focused on eyes and "neglected" the other senses, most of the other species a not so good with vision.
The animal group with right-build eyes are molluscs, they have very good eyes and some are more or less "intelligent" as octopus and in their environment they are very successful. But we did not compete, otherwise we would have lost the struggle ?
not 100%sure, but seems that an nzyme of spermiogenesis is blocked at >35°C
hence, testicles have been fired out and wait for the evolution to allow enter the body for the life party
What, because they (octopi) have eight tentacles, and we only have two arms?
for the first question,
i have an instinct that it is to reduce the intensity of light reaching to photoreceptors which are sensitive to light not to cause any damage. light passes through all the other cell layers before reaching the photoreceptor cells.
such an arrangement is also necessary for photoreceptors to contact with Retinal Pigmented Epithelium (the layer at the back of retina) , these cells controll the exchange of materials between blood supply and photoreceptors; helps the replenishment of photoreceptors. additionally, RPE absorbs excess light energy in order to reduce the scattered light and improve the clarity of images.